Alkali metal modified ferromagnetic chromium oxide, its preparation and use as the magnetic material of recording members



United States PatentO ALKALI METAL MODIFIED FERROMAGNETIC CHROMIUM OXIDE, ITS PREPARATION AND USE AS THE MAGNETIC'MATERIAL OF RE- CORDING MEMBERS Thomas J. Swoboda, Wilmington, Del.,,assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application August-21', 1958 Serial N0. 756,296

22 Claims. (Cl. 252-625) This invention relates to ferromagnetic materials and their preparation. More particularly, it relates to a new type of ferromagnetic chromium oxide, to a method for its preparation and to magnetic recording members having this new type of ferromagnetic chromium oxide as the magnetic track.

Ferromagnetic materials are employed in a variety of applications. For example, these materials are employed in magnetic sound recording tapes, drums and records, memory devices, microwave circuitry, and as magnetic cores such as coil cores in electronic equipment. In some of these applications, especially those requiring magnetic materials of low loss characteristics at high frequencie's, or of relatively high coercive force, ferromagnetic oxides are normally more useful than ferromagnetic metals. Heretofore only cobalt, nickel, or iron alloys or magnetic iron oxides have been used in the manufac ture of magnetic recording tapes.

Among the known ferromagnetic oxides is chromium dioxide. However, the heretofore known forms of this oxide have not possessed the magnetic properties and particle characteristics which would make them useful in certain practical applications, e.g., in the manufacture of magnetic recording tapes, magnetic memory cores for computers, gyrator elements, etc.

Recently a new form of chromium oxide having a combination of properties which makes it particularly useful in various applications has been prepared. This new chromium oxide of small particle size, the average particle length being less than 10 microns, and the crystals having an elongated shape with their length from 2 to 6 times their width and having a tetragonal crystal structure recently has been discovered. This new form of ferromagnetic chromium oxide is more fully described and claimed in U.S. patent application of 'Paul Arthur, Jr., Serial No. 515,521, filed June 14, 1955. Even though these new chromium oxides are especially useful in certain applications, it is still desirable to develop other types of chromium oxides having, more improved magnetic and physical properties. One such type is described in my copending application Serial No. 515,523, filed June 14, 1955, now abandoned, of which the present application is a continuation-impart.

It is an object of this invention to provide a new type of ferromagnetic chromium oxideand a method for its preparation. A furthe object is to provide novel chemically modified ferromagnetic chromium oxides which are especially useful in the manufacture of magnetic recording tapes and in the manufacture of ceramic bodies such as magnetic cores. Another object is to provide a magnetic recording member having a magnetic track comprising this new type of chemically modified ferromagnetic chromium oxide, Other objects will appear hereinafter.

These and other objects'of this invention are accomplished by providing novel ferromagnetic chromium oxides consisting essentially of small acicular particles of tetragonal crystal structure whose average lengthis not more than 10 microns with no more than 10% of the particles being longer than 10 microns, the said oxides containing 59.5% to 61.9% chromium and 0.03% to 1% (30010,000 ppm.) of an alkalimetal as an integral constituent of the crystal lattice. These novel ferromagnetic chromium oxides in which some of the chromium atoms in the crystal lattice are replaced by alkali metal atoms exhibit on X-ray diffraction analysis a pattern corresponding to that of a tetragonal crystal structure having cell constants of a =4.41:0.10A, and c =2.90i0.10A.

The novel chemically modified ferromagnetic chromium oxides of this invention have useful magnetic and physical properties. These new chemically modified ferromagnetic chromium oxides are particularly useful as the magnetic material in magnetic recording members which comprise a carrier, such as a tape, drum or record of non-magnetic material having bonded thereto a magnetic track of magnetic material and a binder therefor.

The process of making the products of-this invention comprises heating chromium trioxide, CrO at a temperature Within the range of 400 to 525 C. under a pressure of at least 500 atmospheres and in the presence of an aqueous alkali metal sulfate solution, and separating and drying the resulting chemically modified chromium oxide.

As indicated above, the ferromagnetic chromium oxides of this invention containing from 0.03% to 1% of their weight of alkali metal atoms as integral parts of the crystal lattice possess a number of. properties or characteristics which make them especially suitable for use in certain applications. In addition to their being of small particle size as defined above, these particular-particles also possess a more acicular, or needlelike, shape than hitherto known chromium oxides. They have an average axial ratio, i.e., the ratio of the length of the-particle to its width or thickness, of at least 2:1. Generally this ratio is at least 8:1 and sometimes it is in the range 10-20z1. This acicular shape makes them especially well adapted for use in coating compositions to be applied on film, tape or other substrate. As a result of this acicular shape, the particles can be oriented in closer relationship during the mechanical spreading of the oxide composition in thin layers on a substrate. This in turn results in more uniform magnetic characteristics ofthe coated substrate. i

The tetragonal crystal structure of the chromium oxides of this invention containing as constituents of the crystal lattice a small proportion of alkali metals as modifiers is of the rutile type, i.e., it has the same type of crystal structure as rutile, TiO Ferromagnetic chromium oxides preparedb y hitherto known published methods have also exhibited in conjunction with other crystal structures the tetragonal crystal structure. However, they have always been associated with the chromium oxides having other crystal structures, e.g., the corundumtype crystal structure of Cr O In contrast, the chemically modified chromium oxides of this invention have only a single crystal structure as shown by their X-ray diffraction pattern. In an ideal tetragonal crystal of chromium dioxide there would be onechromium atom for each two oxygen atoms, hence the chromium content of. such a crystal would be 61.91%. However, the chromium oxides of this invention contain 59.5 to 61.9% chromium and this indicates that there is a chromium deficiency in the crystal lattice, i.e., there are a small number of the chr'omium'atoms required for the ideal tetragonal crystal missing in the products of this invention. In addition to this deficiency of chromium, alkali metals are contained as an integral constituent of the crystal lattice in the products of this invention.

The use of an alkali metal sulfate in the preparation of the modified chromium oxides of this invention has the following advantages: It favors the formation of acicular particles and the formation of products exhibiting high coercive forces.

The process of this invention is conveniently carried out as follows: A corrosion-resistant container, i.e., a container constructed of a material'which is inert to the reactants under the reaction conditions, is used. A very satisfactory container is made of platinum. The container can be a completely sealed tube having flexible walls or it can be any type of vessel permitting transmission of the desired pressure to the reaction system such as a cvlindrical tube closed at one end and equip ed at the other end wi h a closelv fitting piston. The container is char ed with chromium trioxide nd an aqueous so ut n of an a kali metal sulf te, e.g., lithium. sodium, potassium or cesium sulfate. This aoueous solu ion acts as a crvstalliv a on medium and an amount sufficient to control crvstallization of the chromium oxide in it is used, Amounts of amieous alkali metal su fate solution ran ng fr m 0.1 to 1.25 times the weight of the chromium tri xi e char ed are operab e. The use of pro ortions of alka i met l su ate so u ion ran ng from 0.1 to 0.5 t m s t e Weiobt of hromi m trioxide results in be ter vie ds of the chemically modified chromium oxides of this in n ion.

Th conc n ration of t e alkali metal sulfate in the annemis ,qnlntion can b vari d widelv in the rocess of t is i ention. F rrom n ic chrom um oxides having the b e-s eci ed proportions of alkali metal in the cr s l lat ice are obtai ed wi h alka i met l sulfate solu ns ran in from to 50% (by wei h concentr i n. However. it is preferred to use alkali metal sulfate sn 11t nr| o 70% to 30% concentration since these ive i er vields of t e desired chromium oxides. When concen rations of -2 are emp o ed. it is pref rred h t the amo nt of solution be at least 0.2 times the Wei ht of chromium trioxide. Generally concentratrations in this ran e favor the formation of particles having small axial ratios w i e i er concentrationsfavor more acicular particles. Optionally the alkali metal sulfate solution can be acidi ed with a small amount of sulfuric acid. e.g., about 5% based on the total weight of the solution. The presence of sulfuric acid is not essential since chromium oxides of the type defined herein are obtained without it.

After the corrosion-resistant container is char ed with the chromium trioxide and aqueous alkali metal sulfate so ution. it is closed and p aced inside a lar er waterfi ed vessel cap e of withstanding hi h temperatures and pressures. The outer vessel 'is then closed and the water pressure is raised to at least 500 atmospheres. Satisfactorv resu ts are ob ained with pressures ranging from 500 to 3000 atmos heres. Pressures above 3000 atmospheres can be used if the available equipment is strong enough to withstand them. The reaction vessel is then heated to a temperature between 400 and 525 C preferablv between 450 and 475 C. Temperatures above 525 C. are not desirable since they cause de composition of the ferromagnetic chromium oxide.

The reaction times are not critical. Periodsranging from a few minutes, e.g., 5 minutes, to an hour or more at the reaction temperature of 400 to 525 C. are sufiicient. Lon er times are not necessary to form the chemically modified chromium oxides of this invention. Likewise, the time of heating the reaction vessel to the re- I f Y 4 sired operating temperature for the desired time, the entire reaction vessel is cooled with the pressure maintained until room temperature (ca. 25 C.) is reached. The water pressure is then released, and this results in the corrosion-resistant reaction vessel being ruptured by the by-product oxygen present in the container. The resulting, finely divided, black, acicular, chemically modified chromium oxide is separated from a dark-colored aqueous phase by filtration, and is then washed with water and dried. It is often convenient to follow the water washing of the chromium oxide particles by a wash with a water-miscible volatile organic solvent, e.g., acetone, before air-drying the product.

The chromium trioxide and alkali metal sulfate used in the process of this invention can be any of the commercially available materials of good quality. They do not need to be specially purified.

The chemically modified chromium oxides of this invention exhibit several magnetic characteristics which make them especially valuable for use in various applications. Among the magnetic properties of a material which are critical factors in their usefulness in certain applications are the intrinsic coercive force, H and the maximum induction, B Definitions of these magnetic terms and the symbols representing them are defined in Special Technical Publication No. of the American Society for Testing Materials, entitled Symposium on Ma netic Testing (1948), Pages 191-198.

The alkali metal-modified chromium oxides of this invention have a maximum magnetic induction, B of the same magnitude as the best ferromagnetic iron oxides now used in magnetic recording tapes. Although values of B are readily obtained, e.g., by means of a cathode ray magnetization curve tracer as described in detail by Scherb in the Review of Scientific Instruments, 19, 411-- 419 (1948), the saturation per gram or sigma value, a is more convenient for routine determinations (Bozorth, Ferromagnetism, D. Van Nostrand & Co., New York, 1951, pages 7 and 8). The maximum induction and sigma value are related by the equation B H=41rdo' where H is field strength and d is density of sample. The sigma values given herein are determined on apparatus similar to that described by T. R. Bardell on pages 226-228 of Magnetic Materials in the Electric Industry, Philosophical Library, New York, 1955.

Another magnetic property which is greatly desired for materials used in certain applications is the intrinsic coercive force, H The alkali metal-modified chromium oxides of this invention exhibit coercive forces significantly greater than the coercive'forces of the chromium oxides prepared in the presence of water only as described in the aforementioned application Serial No. 515,521. The intrinsic coercive forces of the products of this invention range from at least 60 to about oersteds. Ferromagnetic oxides having such intrinsic coercive forces are especially suitable for use in magnetic recording tapes, and in focusing magnets. The values for the intrinsic coercive forces given herein are determined by a modified form of the apparatus described by Davis and Hartenheim in the Review of Scientific Instruments, 7, 147 (1936).

The invention is illustrated in further detail by the following examples in which the proportions of ingre- Clients are expressed in parts by weight unless otherwise noted.

Example I A platinum tube having flexible walls is charged with 5.00 parts of chromium trioxide, 1.25 parts of water and 0.31 part of sodium sulfate. The charged tube is sealed and placed in a water-filled metal bomb where it is heated to 450 C. and 1000 atmospheres pressure. After holding at this temperature and pressure for 1 hour, the reactor is cooled and'depressured. The platinum tube is opened carefully to release the oxygen formed as a byproduct of the reaction. A black, finely divided, strongly '0.31 part of sodium sulfate.

5 magnetic powder isseparated from a very darkflambercolored liquid by filtration. The solid is washed with water and then with acetone and is finally air-dried.

There is isolated 3.62-parts of chromium oxide needles, and about 0.2 part of chromium oxide needles remains adhering to the walls of the tube. The total yield amounts to about 90% of theory. This acicular chromium oxide has an intrinsic coercive force of 88 oersteds. This ferromagnetic sodium-modified chromium oxide has a rutile-type crystal structure with cell constants of a =4.41A. and c =2.92A. The particles exhibit an X-ray diffraction pattern having the reflection lines listed 1 inthe following table. In this table the columnheaded 1 gives the interplanar spacings in Angstrom units, and the column headed I gives the relativereflection intensities.

d I d I d I strong. 1.321.... medium. 0.881.... faint. strong. 1.317 medium. 0.873.". weak. faint. 1.21 faint. 0.867.. faint.

medium. l.13 faint. 0.847."; weak. weak. 1.10.-. faint. 0.820 weak. strong. 1. weak. 0.811 Weak. medium. 1. weak. 0.790.... weak.

weak. 0. faint.

faint. 0. faint.

Example 11- Another sample of sodium-modified chromium oxide is prepared by the-procedure described in Example I, employing a charge consisting of 5.00 parts of chromium trioxide, 1.25 parts of 5% aqueous sulfuric acid, and Heating is carried out at 450 C. under 1000 atmospheres pressure for 1 hour. The resulting acicular ferromagnetic chromium oxide is found to contain 59.65% chromium, and 800 4000 p.p.m. of sodium. The product has an intrinsic coercive force, Hci, 0f 77 oersteds.

Examination of the chromium oxide crystals by electron microscope at a magnification of 2400X indicates that individual needles have lengths ranging from' 0.28 microns. The particles are acicular and have length-towidth ratios ranging from about 4:1 to about 20:1, the average being about 10:1. Thev individual needles form aggregates 2 to 4 microns across.

A dispersion of these chromium oxide particles with a nitrocellulosebinder is spread in a uniform'layer on a cellulose acetate tape.' The resulting tape-after coating and drying hasa total thickness ofabout 2 mils,- and the thickness of the magnetic coating is about 0.5 mil. The coercive -force of the magnetic coating "on the'tape is found to be 60 oersteds. Thistape is suitable for-use as a magnetic recording tape.

Example III T r A platinum tube with flexible. walls is'charged with 5.00 parts of chromium trioxide, 1.25 parts of 5% aqueous sulfuric acid and 0.31 part of sodium'sulfate. The

tube is sealed and thenheated at 450C. at a pressure of 750 atmospheres for one hour.

On working up thereaction'product as in Example I, there is isolated 3.72 parts ofacicular chromium oxide with about 0.2 part of product'still adhering to' the walls of the tube. The totalyield corresponds to about 93% of the theoretical.' The needlelikecrystals are.-aggregated into clumps of about '2 to 4 microns in diameter, with the length of the individual needles ranging from 0.2 to 8 microns and being 3 to 12 times the width. The average length-to-width ratio is about 85.1. This product contains 60.07% chromium and 0.1-1.0% (1000l0,000 p.p.m.) of sodium. It has an intrinsic coercive force of 84' oersteds.

Example IV parts of wa't'en The tube is sealed and heated to 450 C. under 1000 atmospheres pressure for 0.5 hour astrr the. preceding examples. "After the tube iscooled, depressured, and opened, there is isolated byfiltration, washing, and drying 3.57 parts of black, finely divided, acicular chromium oxide. On analysis this .chromium oxide is found to contain 1000-5000 p.p.m. (0.1-0.5%) potassium. This chromium oxide has an intrinsic coercive force, H of 113 oersteds.

"Upon examination under an electron'microscope, this product is foundto consist of needlelike particles of 0.16 microns in'leng'th with length-to-width ratios of 2:1,t'o10z1 (average 8:1). 7

Another sample of potassium-modified chromium oxide is prepared by the procedure described above withthe exception that the chromium trioxide and potassium sulfate-solution are heated at 450 C.'and l000 atmospheres pressure for one hour. The product is a finely divided ferromagnetic chromium oxide 'of acicular particle shape and contains 60.04% chromium.

Example V Asodium-modified ferromagnetic chromium oxide is prepared according to the procedure of Exainple'I except in that example.

chromium trioxide, 0.32 part of potassium sulfate, and

.drate, and 1.25 parts of water.

- gram.

that'thequantity of sodium sulfate is reduced to 0.08 part. The product is a black acicular chromium oxide having an intrinsic coercive force,'H of 66 oersteds and a sigma value, a of 82 gauss cm. /gram.-- The particles range in length from about 0.8 to about 7 microns and have axial ratios in the range of 2:1 to 5:1. The average axial ratio is'about 3:1.

' I Example VI flexible platinum'tube is charged with 5.00 parts of chromium trioxide, 0.36 part of lithium sulfate monohy- The tube is sealed, heated under the conditions given in Example IV for 0.5 hour, and the reaction product is worked up as "described There is obtained 3.58 parts of finely divided. chromium oxide containing 60.31% chromium, and400-2000 p.p.m. of lithium; This product has"an intrinsic coercive force of oertsteds and a sigma value,

a of 72 gauss cmL /gram. Particle lengths range from about 0.5 to about.8 microns, and length-to-width ratios from about 3:1 to about 20:1 (average about 8:1).

Example Vll A platinum tube with flexible walls is charged with 5.00 parts of chromium trioxide, 2.37 partsof water,

cm. /gram. The particles range in length from 0.1 to

10 microns and have an average axial ratio of about 2:1. The product is shown by analysis to contain 800-5000 p.p.m. of potassium.

Example VIII A flexible-walled platinum tube is charged with 5.00 parts of chromium trioxide, 0.63 part of cesium sulfate, and 1.25 parts of; water. The tube is sealed and heated at 450) C. under 750- atmospheres pressure for one hour. The product has an intrinsic coercive force H of 123 oersteds and a sigma value, a of 68 gauss cm. It is a black cesium-modified chromium oxide and is shown by analysis to contain 0.2-1.0% cesium.

The particles range from 0.5 to 6 microns in length and exhibit an average axial ratio of about 6: 1.

The process of the present invention, involving the use of alkali metal sulfate solution during the heating step, results in the formation of acicular crystals of chromium oxide, and the introduction of small proportions of alkali metals into the crystal lattice of the chroimum oxide. The products of the present invention are particles having an average length not over-- microns and having average axial ratios ranging from about 2:1 to about :1 or more. Products having an axial ratio of at least 8:1 are particularly well adapted for applications where it is desired to orient the magnetic particles in a coating with the'particles substantially all in parallel relationship.

forces are particularly desirable include magnetic recording tapes, drums and records, and focusing magnets.

The alkali metal-modified chromium oxides of this invention are also useful in a Wide variety of other applications, especially in the manufacture of ceramic bodies such as magnetic cores. More particularly they are useful in magnetic memory cores for computers, in-microwave attenuators, in gyrator elements, in electrically operated high frequency switches, in low-loss transformer cores for megacycle/second frequency ranges, and in magnetic clutches.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as .defined in the appended claims.

The embodiments'of the invention in which an exclusive property or privilege is claimed are defined as follows; 1

1. A modified-ferromagnetic chromium 'oxideconsisting essentially of small acicular particles of tetragonal crystal structure having an average length of not more than 10 microns withno more than 10% of theparticles being longer than 10 microns, the said oxide containing 59.5% to 61.9% by weight of chromium and 0.03% to 1% by Weight of an alkali metal as an integral constituent of the crystal lattice.

2. A modifiedferromagnetic chromium oxide consisting essentially of small acilcular particles of tetragonal crystal structure having a ratio of. length to width .of at least 2:1 and an average length of not more than 10 microns with no more than 10% of the particles being longerv than 10 microns, :thesaid' oxide containing 59.5 to 61.9% by weight of chromium and 0.03%. to 1% by weight of sodium as an-integral constituent of the crystal lattice.

3. A modified ferromagnetic chromium oxideconsisting essentially of-small acicular particles of tetragonal 'crystal structure having a ratio of length to 'Width of at least 221 and 'an average length of not more than 10 microns 'with'no more than 10% of the particles being longer than '10 microns, the said oxide contai'ning'*59.5% to 61.9% by weight ofchromium'and 0.03% to 1%. by weight of potassium as an integral constituent ofthe crystal lattice. a 1

4. A modified ferromagnetic chromium oxide consisting essentially of small acicular particles of tetragonal crystal structure having a ratio of length to Width of at least 2:1 and an average length of not more than 10 microns with no more than 10% of the particles being longer than 10 microns, the said oxide containing59 .5% to 61.9% by weight of chromium'a nd 0.03% to 1% by Weight of'lithium as'an integral constituentof the crys tal lattice.

{5; A modified ferromagnetic chromium oxide consisting essentially of small acicular particles of tetragonal crystal structure'having a ratio of length to jwidth of at least 2:1 and an" average length of not'more than 10 microns with no more than 10% of the particles being longer than 10 microns, the said oxide containing 59.5%

'to. 6l.9 by weight of chromium and 0.03% to 1% by weight of cesiumas an integral constituent of the .crystal lattice 6. A modified ferromagnetic chromium'oxide consisting essentially of small acicular particles of tetragonal crystal structure having an average ratio of length to Width of at least 8:1 and an average length of not more than 10 microns with; no more than 10 of the particles being longer than 10 microns, the said oxide containing 59.5% to 61.9% by Weight of chromium and 0.03% to 1% by Weight of an alkali metal as an integral constituent of the crystal lattice.

7. A magnetic recording member comprising a carrier of non-magnetic material having bonded thereto a magnetic track of a modified ferromagnetic chromium oxide consisting essentially of small acicular particles of tetragonal crystal structure having an average length of not more than 10 microns with no more than 10% of the particle being longer than 10 microns, the said oxide containing 59.5 to 61.9% by weight of chromium and 0.03% to 1% by Weight of an alkali metal as an integral constituent of the crystal lattice.

8. A magnetic recording member comprising a carrier of non-magnetic material having bonded thereto a magnetic track of a modified ferromagnetic chromiurri oxide and a binder therefor, said modified ferromagnetic chromium oxide consisting essentially of small acicular particles of tetragonal crystal structure having a ratio of length to Width of at least 2:1 and an average length of not more than 10 microns with no more than-10% of the particles being longer than 10 microns,'the said oxide containing 59.5% to 61.9% by weight of'chromium and 0.03% to 1% by Weight of sodium as an integral constituent of the crystal lattice.-

9. A magnetic recording member comprising a carrier of non-magnetic material having bonded thereto a*mag-' netic track of a modified ferromagnetic chromium oxide and a binder therefor, said modified ferromagnetic chromium oxide consisting essentially of small acicular particles of tetragonal crystal structure having a ratio of length to width of at least 2:1 and an average length of not more than 10 microns with no more than 10% of the particles being longer than 10 microns, the said oxide containing 59.5% to 61.9% by weight of chromium and 0.03% to 1% by weight of potassium as an integral constituent of the crystal lattice.

10. A magnetic recording member comprising a carrier of non-magnetic material having bonded thereto a magnetic track of a modified ferromagnetic chromiumoxide and a binder. therefor, said modified ferromagnetic chromium oxide consisting essentially of small acicular particles of tetragonal crystal structure having a ratio of length to width of at least 2:1 and an average length of not more than-l0 microns with no more than 10% of the particles being longer than 10 microns, the said oxide containing 59.5 to 61.9% by weight of chromium and 0.03% to 1% by Weight of lithium asian integral constituent of the crystal lattice.

11. A magnetic recording member comprising a carrier of non-magnetic material having bonded thereto a magnetic track of a-modified ferromagnetic chromium oxide stituentof the crystal lattice.

12. Amagnetic recording member comprising a carrier of non-magneticmatefial having bonded thereto a magnetic track ofamodified ferromagnetic chromium oxide and a binder therefor, said modified ferromagnetic chromium oxide consisting essentially of small acicular particles of tetragonal crystal structure having an average ratio of length to width of at least 8:1 and an average length of not more than 10 microns with no more than 10%1ofthe particles being longer than-10 microns, the

said oxide containing 59.5% to 61.9% by weight of chromium and 0.03% to 1% by weight of an alkali metal as an integral constituent of the crystal lattice.

13. Process for preparing a modified ferromagnetic chromium oxide which comprises heating chromium trioxide in an aqueous alkali metal sulfate solution at a temperature within the range of 400 to 525 C. under a pressure of at least 500 atmospheres, and separating and drying as the resulting product a modified ferromagnetic chromium oxide containing 59.5% to 61.9% by Weight of chromium and 0.03% to 1% by Weight of an alkali metal as an integral constituent of the crystal lattice.

14. Proces for preparing a modified ferromagnetic chromium oxide which comprises heating chromium trioxide in an aqueous sodium sulfate solution at a temperature within the range of 400 to 525 C. under a pressure of at least 500 atmospheres, and separating and drying as the resulting product a modified ferromagnetic chromium oxide containing 59.5% to 61.9% by weight of chromium and 0.03% to 1% by weight of sodium as an integral constituent of the crystal lattice.

15. Process for preparing a modified ferromagnetic chromium oxide which comprises heating chromium trioxide in an aqueous potassium sulfate solution at a temperature within the range of 400 to 525 C. under a pressure of at least 500 atmospheres, and separating and drying as the resulting product a modified ferromagnetic chromium oxide containing 59.5% to 61.9% by weight of chromium and 0.03% to 1% by weight of potassium as an integral constituent of the crystal lattice.

16. Process for preparing a modified ferromagnetic chromium oxide which comprises heating chromium trioxide in an aqueous lithium sulfate solution at a temperature within the range of 400 to 525 C. under a pressure of at least 500 atmospheres, and separating and drying as the resulting product a modified ferromagnetic chromium oxide containing 59.5% to 61.9% by weight of chromium and 0.03% to 1% by weight of lithium as an integral constituent of the crystal lattice.

17. Process for preparing a modified ferromagnetic chromium oxide which comprises heating chromium trioxide in an aqueous cesium sulfate solution at a temperature within the range of 400 to 525 C. under a pressure of at least 500 atmospheres, and separating and drying as the resulting product a modified ferromagnetic chromium oxide containing 59.5% to 61.9% by weight of chromium and 0.03% to 1% by weight of cesium as an integral constituent of the crystal lattice.

18. Process for preparing a modified ferromagnetic chromium oxide as set forth in claim 13 in which said chromium trioxide is heated in an aqueous alkali metal sulfate solution at a temperature within the range of 450 to 475 C. under said pressure of at least 500 atmospheres. I

19. Process for preparing a modified ferromagnetic chromium oxide as set forth in claim 13 in which said chromium trioxide is heated in an amount of aqueous alkali metal sulfate solution of from 0.1 to 1.25 times the weight of said chromium trioxide and said aqueous alkali metal sulfate solution contains from 4 to by weight of alkali metal sulfate.

20. Process for preparing a modified ferromagnetic chromium oxide as set forth in claim 13 in which said chromium trioxide is heated in an amount of aqueous alkali metal sulfate solution of from 0.1 to 0.5 times the weight of said chromium trioxide and said aqueous alkali metal sulfate solution contains from 20 to 30% by weight of alkali metal sulfate.

21. Process for preparing a modified ferromagnetic chromium oxide as set forth in claim 13 in which said aqueous alkali metal sulfate solution contains a small amount of sulfuric acid.

22. Process for preparing a modified ferromagnetic chromium oxide as set forth in claim 13 in which the reaction mixture, after heating at said temperature of 400 to 525 C. under said pressure of at least 500 atmospheres, is cooled to room temperature of about 25 C. while maintaining pressure thereon of at least 500 atmospheres.

References Cited in thefile of this patent UNITED STATES PATENTS Camras Nov. 16, 1954 Camras Jan. 11, 1955 OTHER REFERENCES 

1. A MODIEFIED-FERROMAGNETIC CHROMIUM OXIDE CONSISTING ESSENTIALLY OF SMALL ACICULAR PARTICLES OF TETRAGONAL CRYSTAL STRUCTURE HAVING AN AVERAGE LENGTH OF NOT MORE THAN 10 MICRONS WITH NO MORE THAN 10% OF THE PARTICLES BEING LONGER THAN 10 MICRONS, THE SAID OXIDE CONTAINING 59.5% TO 61.9% BY WEIGHT OF CHROMIUM AND 0.03% TO 1% BY WEIGHT OF AN ALKALI METAL AS AN INTEGRAL CONSTITUENT OF THE CRYSTAL LATTICE. 